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Why is the Water Warm One Day and Cold Another?
People often ask the FRF staff why water temperatures change so rapidly on the Outer Banks -- not an easy question to answer since the ocean currents of this region are very complex. This area is the southern most extent of cold and relatively low saline Labrador Current water that travels south along the coast, then mixes with the warm higher saline water of the northerly flowing Gulf Stream. As the Gulf Stream passes Cape Hatteras, it moves in a northeast direction, sometimes shedding rings, or eddys, of warm water that mix with the coastal water. Satellite imagery of sea surface temperature is useful in identifying different water masses. Water flowing out of the Chesapeake Bay also has an influence on our coastal waters. As low saline water leaves the bay, it is less dense than the ocean water and tends to float on the surface. This water may flow south down the coast to the Outer Banks, most likely when there is a wind from the north. Coriollis force, which causes currents to bend to the right of their direction of travel in the northern hemisphere, will help keep this water against the coast. These flows are often visible as a coastal front traveling south. In addition, the wind has a dramatic effect on driving currents. Generally the coastal water has warmer (less dense) water on the surface and cooler (more dense) water below. A wind blowing offshore can push the surface water along with it; colder water from below replaces the surface water, a process called upwelling. Conversely, an onshore wind can transport the warm surface water shoreward pushing the cold water below, a process called downwelling.
End-of-Pier Water Temperature and WindsSix temperature sensors are used to monitor the water temperature at the end of the FRF pier (details). The plots below show water temperature for 15 days (horizontal axis) from the surface to 7m depth (vertical axis).
Wind vectors are computed
in FRF coordinates from an anemometer (gauge 932) located at the end of
the pier and are shown below with water temperature profiles for 1-15
July 2002 and 2003.
Note the persistence of offsore wind between 3-14 July 2003 producing an upwelling event and cooler water temperatures. A slower wind during 9-10 July allowed the surface waters to warm. A weak offshore wind may not be able to overcome the buoyant forces of the stratified waters (less dense warm water over denser cold water) and the warm water could actually move back onshore. If warm water transport offshore continues long enough the water column may become nearly uniform in temperture and the buoyant force reduced, and the warm surface water if farther offshore an will take longer for an onshore wind to transport back to the coast.
The next two weeks of July 2002 show relatively warmer water than the
same time in 2003, with three short
term offshore wind events that caused a little cooling, the most obvious
on the 24th and 25th. Note that
onshore winds rapidly warm the entire water column (21-22, 25, and 31
July 2002).
The last two weeks of July 2003 experienced little onshore wind and
atypical cooler water for this time of year.
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